Process of preparing wax modified polystyrene compositions



United States Patent PROCESS OF PREPARING WAX MODIFIED POLYSTYRENECOMPOSITIONS No Drawing. Application June 14, 1954 Serial No. 436,696

4 Claims. (Cl. 260-285) This invention is concerned with theincorporation of waxes in polystyrene molding compositions andparticularly those compositions adapted for the molding of phonographrecords.

In copending application, Serial No. 82,908, filed March 22, 1949, sinceissued as Patent No. 2,681,323, it has been disclosed that records ofgood sound quality and wear resistance are obtained when the amount ofwax incorporated in a suitable polystyrene exceeds the amount that canbe retained in solid solution by the polystyrene and that the amount ofwax incorporated should be such that the resulting mixture yields astable dispersion of the wax throughout the polystyrene mass, which canbe molded without the wax migrating or sweating out to the mold surface.

To obtain stable dispersions of wax and polystyrene according tocopending application Serial No. 82,908 it is necessary to intensivelymix the wax and polystyrene at temperatures from 160 C. and up to 225 C.and higher. Moreover, despite these high temperatures, it took as longas an hour and even more with equipment such as mixing rolls to obtain astable dispersion of wax and polystyrene.

The present application is a continuation-in-part of our copendingapplication Serial No. 316,072, filed October 21, 1952, now Patent No.2,779,744, in which there is disclosed wax modified polystyrenecompositions prepared by mixing together an aqueous wax emulsion and anaqueous polystyrene emulsion.

According to the present invention, wax modified polystyrenecompositions capable of being molded without wax sweatout are preparedby mixing together suitable proportions of an aqueous wax emulsion andof an aqueous polystyrene emulsion, coagulating the mixture to form adehydrated composite wax-polystyrene coagulum, and then subjecting thecoagulum to mechanical working or mixing at temperatures from about 160C. to 225 C. or higher if desired. The mechanically worked coagulum canthen be comminuted, or diced for convenient use as molding material, andupon being either injection or compression molded, yields phonographrecords free from wax sweatout and ofgood sound quality and wearresistance.

Coagulation of the mixed wax and polystyrene emulsion can be effected bythe addition of conventional precipitation agents for polystyreneemulsions such as aqueous sodium chloride-hydrochloric acid solution,isopropanol water solution or acetic acid-water solution. The coagulumthus obtained is dehydrated as by exposure to air, or more rapidly bybeing moderately heated in ovens or on drum dryers at temperatures up toabout 150 C.

The mixture of wax emulsion and polystyrene emulsion'can also becoagulated by heating the mixture as on drum driers and concurrentlydehydrated thereon.

Surprisingly, the mechanical working or mixing of the dried compositewax-polystyrene coagulum at 160 C.

or higher temperatures to form compositions characterized by freedomfrom wax sweatout when molded can be carried out in a fraction of thetime and advantageously with simpler mixing equipment than is requiredto form by mixing a comparable moldable composition from wax andpolystyrene in bulk or granular form (e. g. 10 to mesh).

For example, with heated mixers of the screw extrusion type which as aclass do not perform as an intensive mixing function as a Banbury typemixer, a single passage through such extruder of a composite dehydratedwax-polystyrene coagulum is ordinarily adequate to obtain a moldablewax-polystyrene dispersion free from wax sweatout when molded. On theother hand, a mechanical mixture of commercial granulated polystyreneand comminuted wax does not form a homogeneous stable dispersion evenafter repeated passages through'a screw extruder, since under theinfluence of heat, the wax quickly melts, separates out, and therebycoats the extruder screw surfaces with a lubricating film that preventsetfective shear development necessary for good mixing.

On heated two or multi-parallel roll mixing equipment, it usuallyrequires from one to two hours of mixing to obtain from a mixture ofgranular polystyrene and comminuted wax, a satisfactory moldablewax-polystyrene dispersion. In contrast, from 2 to 8 minutes mixing.

time on heated rolls is usually adequate for producing a satisfactorywax-polystyrene dispersion, from a dehydrated wax-polystyrene coagulum.

While the intensive mixing action of a Banbury can effect a satisfactorydispersion of wax and granulated polystyrene in about fifteen minutes,this time can be reduced in half when processing dehydratedwax-polystyrene coagulums.

Although it is not known with certainty, nevertheless it is believedthat in a dried coagulum of wax and polystyrene the wax particles arerelatively strongly physically attached to polystyrene particles, andthat accordingly upon heating such coagulums to mixing temperatures ofabout C. and higher, the wax particles do not readily flux together, atleast not in an amount tending to form a continuous liquid wax phasewhich would tend to delay formation of a satisfactory dispersion of waxin the polystyrene.

'Moreover, it has been found possible to prepare dried compositecoagulums of wax and polystyrene containing more wax than is ordinarilydesired in the final composition for molding phonograph records and toreadily mix such high wax-containing composite coagulant with additionalwax-free polystyrene as well as conventional mineral fillers at mixingtemperatures from about 160 C. to 225 C. and higher to form satisfactorywax-polystyrene dispersions containing the desired amount of wax, e. g.from 2 to 10 percent by weight of the total polystyrene content as isdescribed in copending applications Serial No. 82,908 and Serial No.316,072.

For mixing with wax-free polystyrene, it is possible to use compositedehydrated wax-polystyrene coagulums containing up to about 25% byweight of wax, and still obtain quicker and better compounding ofwax-polystyrene molding compositions suitable for phonograph recordmanufacture, than would be the case by directly mixing the same ultimateproportions of wax and granulated polystyrene.

The polystyrene emulsions may be prepared by known methods of emulsionpolymerizing styrene which are controlled so as to yield a polystyrenehaving average relative molecular weights by the Staudinger method offrom 40,000 to about 150,000 and preferably from about Relativeviscosity Time for solution to discharge from pipette Time for solventto discharge from pipette using a solution containing 0.1000 gram ofpolymer per 100 milliliters of benzene free from thiophene, measurementsbeing made at 20 C.

Specific viscosity =relative viscosity 1 Molecular weight (57,800)(specific viscosity) C=0.1000 gm. per 100 ml.

According to this method, polystyrenes having a specific viscosity of0.865 have a calculated average molecular weight of 50,000 andcorrespondingly polystyrenes having a specific viscosity of 1.38 have acalculated average molecular weight of 80,000.

The aqueous wax emulsions can be prepared according to methods wellknown to the art.

The following examples illustrate a typical preparation of a polystyreneemulsion, all parts being by weight in it and subsequent examples.

Example 1 Fifty parts of styrene monomer, 75 parts filtered water, 1.5parts emulsifier such as Dresinate 731 which is the sodium soap ofdehydrogenated rosin. .02 part of a polymer modifier such as a tertiarydodecyl mercaptan and .01 part sodium hydroxide are charged to a mixingtank under an atmosphere of nitrogen, heated to 40 C.-50 C. andthoroughly emulsified by high speed agitation. One-tenth part of acatalyst such as potassium persulfate is then added and the mixtureheated to 80 C.85 C. and maintained at this temperature untilpolymerization is complete. This is determined by sampling the reactionmixture and determining the solids content. The average particle size ofthe polystyrene was 0.25 micron, being substantially free of particleslarger than 0.5 micron.

To prepare a wax emulsion the following typical procedure may beemployed.

Example 2 Six parts of microcrystalline wax, 1.00 part zinc stearate,0.35 part polyethylene glycol oleyl ether (Antarox-DlOO sold by GeneralAniline & Film Corp.) and 29.40 parts filtered water are charged to ajacketed kettle, heated to 98 C.99 C. and stirred thoroughly untilemulsified. The wax particle size ranged from 0.005 to 0.12 micron.

To prepare the polystyrene-wax coagulum the following typical proceduremay be employed.

Example 3 Three and five-tenths parts of the emulsion prepared inExample 1 are mixed with 0.5 part of the emulsion from Example 2 and theresulting mixture fed to drum drier where it is dried at a temperatureof about 135 C.145 C. The dried coagulum is removed from the drum insheet form, ground and then fed to a twin screw extruder heated to atemperature of about 160 C. It is extruded in the form of a sheet anddiced. The resulting diced product had a composition of about six partswax for every 100 parts polystyrene. The diced product formedsatisfactory records free from wax sweatout when injection molded.

For some purposes it is desirable to introduce as much wax as possiblein the emulsion stage and in that manner prepare a dehydratedwax-polystyrene coagulum having a high wax content. Such a coagulumconstitutes a master batch which can be reduced in wax content bycompounding aportion of it with the requisite amount of polystyrene toyield a product containing the desired amount of wax for moldingphonograph records.

This procedure facilitates the production of a master batchpolystyrene-wax formulation which can then be readily and quicklyblended with further quantities of polystyrene, fillers, coloring agentsand the like in simple mixing equipment such as heated extruders and thelike.

Example 4 -10-100 mesh. The ground material contained twelve parts ofwax for every parts polystyrene. Then 100 parts of this material weredry blended as by tumbling with 100 parts wax-free polystyrenegranulated to molding material size particles and of about 55,000average molecular weight, 2 parts carbon black, and 0.01 part of 0.1 to10 micron size particles of silicon carbide, the sharp edges of theseparticles having been previously dulled by milling as described in ourcopending application Serial No. 316,072, filed October 21, 1952. Thedry blend was then passed through a heated twin-screw extruder to effectby fiuxing and mixing an intimate dispersion of all the components, thecomposition as extruded having a temperature between 170 C. and 190 C.The extruded material was ground to molding material fineness and thenmolded by means of heat and pressure in phonograph records of goodplaying quality and free from wax sweatout.

The waxes used in the composition can be the natural waxes, particularlythe ester type waxes, for example carnauba, candellila, montan, esparto,ouricury, beeswax, cotton wax, insect or Chinese wax and the like.Another class of ester type waxes are Japan wax, myrtle wax and thesynthetic waxes obtained by substantial hydrogenation of vegetable,marine and animal oils. Another class of natural waxes are the mineralwaxes such as paraffin waxes, microcrystalline waxes, ozokerite andceresin.

The maximum amount of wax which can be incorporated with polystyrene tocomprise a master batch is about 25 parts to each 100' parts ofpolystyrene. Compositions containing wax in excess of this amount becomeincreasingly difficult to compound although it is possible as by specialtechniques such as slowly adding increments of the polystyrene-waxcoagulum to fluxed polystyrene to form satisfactory dispersions.

Fillers maybe added to the dehydrated wax-polystyrene coagulum forreducing the cost and to prevent warpage of records on storage atelevated temperatures. The preferred fillers are diatomaceous silica,slate flour and calcium carbonate.

Coloring matter such as carbon black and pigment can be added to theWax-polystyrene coagulums. The addition of metal soaps such as zincstearate, cadmium stearate and the like are eifective in impartingresistance to needle wear.

Phonograph records of consistently good properties may be prepared bypreforming the mechanically worked wax-polystyrene coagulums intopreforms of the required weight, heating the preform on a table or in anoven to about 140 C., charging the heated preform into the mold orstamper when it is at a temperature of about C., compressing the preformin the mold at a pressure between 2000 and 4000 pounds per square'inchwhile raising the mold temperature to between C. and C. and then coolingthe mold and its contents to about 60 C. to 80 C. before opening thepress. Heated preforms for compression molding can also be formed byheatplasticizing the wax-polystyrene coagulum in a heat injection typecylinder, ejecting a measured amount of the hot material into a transfercup for delivery to a compression molding press. In such operation theplastic is discharged in the cup usually between 170 C. and 200 C.material temperature.

The dehydrated wax-polystyrene coagulums after compounding at 160 C. orhigher are especially adapted, however, to injection molding. In thiscase, dried granules of the composition are fed to the hopper of aninjection machine from which they pass to a heating chamber torpedowhere they are heated to a temperature of about 200 C. and the plasticmass is then injected into a cooled mold at a pressure of 10,000 poundsto 20,000 pounds.

This application is a continuation-in-part of our copending applicationSerial No. 316,072, filed October 21, 1952, and entitled Sound Record.

What is claimed is:

1. Process for preparing wax-containing polystyrene compositions whichcomprises mixing together an aqueous Wax emulsion and an aqueouspolystyrene emulsion in proportions corresponding to between 2 and partswax per 100 parts polystyrene coagulating the mixture, dehydrating themixed coagulum of wax and polystyrene, and then mechanically working thedehydrated coagulum at elevated temperature to form a wax-polystyrenedispersion characterized by freedom from wax-sweatout when molded underheat and pressure.

2. Process for preparing wax-containing polystyrene compositionssuitable for molding of phonograph records which comprises mixingtogether an aqueous wax emulsion and an aqueous polystyrene emulsion inproportions not exceeding about parts wax per 100 parts polystyrene,coagulating the mixture, dehydrating the mixed coagulum, and thenmechanically working the dehydrated coagulum at an elevated temperatureand mixing the Worked dehydrated coagulum with additional polystyrene ata temperature of at least about 160 C. to form a polystyrene compositioncontaining between 2 and 10 parts wax per parts polystyrene andcharacterized by absence of wax-sweatout when molded under heat andpressure.

3. Process for preparing a wax-containing polystyrene compositionsuitable for molding phonograph records which comprises mixing togetheran aqueous natural wax emulsion and an aqueous emulsion of polystyrenehaving an average molecular weight between 50,000 and 80,000 asdetermined by viscosity measurements; coagulating the mixture,dehydrating the mixed coagulum of wax and polystyrene, and thenmechanically working the dehydrated coagulum at temperatures of at leastabout C. to form a wax-polystyrene dispersion characterized by absenceof wax-sweatout when molded under heat and pressure.

4. Process for preparing wax-containing polystyrene compositionssuitable for molding of phonograph records which comprises mixingtogether an aqueous wax emulsion and an aqueous polystyrene emulsion inproportions not exceeding about 25 parts wax per 100 parts polystyrene,coagulating the mixture, dehydrating the mixed coagulum, and thenmechanically working the dehydrated coagnlum at an elevated temperaturewhile mixing the dehydrated coagulum with additional polystyrene at atemperature of at least about 160 C. to form a polystyrene compositioncontaining between 2 and 10 parts wax per 100 parts polystyrene andcharacterized by absence of wax-sweatout when molded under heat andpressure.

References Cited in the file of this patent UNITED STATES PATENTS2,127,381 Herrmann et a1 Aug. 16, 1938 2,540,996 Ryden Feb. 6, 19512,614,093 Wheelock Oct. 14, 1952 2,681,323 Grofi et al. June 15, 19542,695,277 Pabst et a1. Nov. 23, 1954

1. PROCESS FOR PREPARING WAX-CONTAINING POLYSTYRENE COMPOSITIONS WHICHCOMPRISES MIXING TOGETHER AN AQUEOUS WAX EMULSION AND AN AQUEOUSPOLYSTYRENE EMULSION IN PROPORTION CORRESPONDING TO BETWEEN 2 AND 10PARTS WAX PER 100 PARTS POLYSTYRENE COAGULATING THE MIXTURE DEHYDRATINGTHE MIXED COAGULUM OF WAX AND POLYSTYRENE, AND THEN MECHANICALLY WORKINGTHE DEHYDRATED COAGULUM AT ELEVATED TEMPERATURE TO FORM AWAX-POLYSTYRENE DISPERSION CHARACTERIZED BY FREEDOM FROM WAX-SWEATOUTWHEN MOLDED UNDER HEAT AND PRESSURE.